This invention relates to improved piezoelectric moisture analyzers. The analyzers of this invention are particularly useful for moisture measurements in gas streams which also contain acid gases.
Piezoelectric moisture analyzers are well-known instruments. Their construction and use are described in a series of patents to W. H. King, Jr., beginning with U.S. Pat. No. 3,164,004 and a patent to H. M. Crawford, U.S. Pat. No. 3,327,519. In these devices a piezoelectric crystal such as quartz is coated with a substance which is responsive to moisture. Useful materials include polymers such as the sulfonic acid and sulfonate salts of polystyrene which reversibly form hydrates when exposed to moisture. The change in the mass of the coating on the crystal causes a measurable change in the frequency of oscillation of the crystal in an electrical oscillation circuit. This change, which can be electrically detected, provides a measure of the moisture content of the environment in which the coated crystal is placed.
It has been recognized that moisture detectors of the type described above suffer a loss of sensitivity when the gas stream to be analyzed contains an acid gas in addition to moisture. That is, when an acid gas is present in the gas stream, the amount of moisture which can be detected decreases, and the change in the frequency response of the coated crystal to a constant moisture concentration decreases. This loss of sensitivity has been observed whether the polymer coating on the crystal is in the form of a sulfonic acid or a sulfonic acid salt. The loss in sensitivity due to the presence of acids in gases has limited the usefulness and accuracy of piezoelectric moisture analyzers in industrial gas streams containing acids. One example of this problem is in the measurement of moisture in natural gas which frequently also contains H.sub.2 S.
It has now been found that the piezoelectric moisture analyzers of the prior art can be improved to be more stable in gas streams in which an acid gas is present. The coated crystals are stabilized by contacting them with an acid gas for a sufficient period of time to reduce the sensitivity to moisture of the coated crystal to a range in which the stability is no longer affected by the acid gas. Typically, the frequency response of the crystal in the region of stable sensitivity is about 30 to 50 percent lower than the original level of the coated crystal. The acid gas used to stabilize the coated crystal need not be the same as the acid gas found in the gas stream in which the moisture is to be measured. However, it should be recognized that the gas used to stabilize the coated crystal must be selected so that the coated crystal after stabilization will not react with the acid gas of the gas stream to be measured. If such a reaction were to occur, there would be a further alteration of the sensitivity of the coated crystal. Thus, it is preferred that the gas chosen to stabilize the crystal is the same as the gas that is found in the gas stream which will be monitored by the instrument. The stabilization of the coated crystals occurs faster if the acid gas used is moist with water vapor or, is saturated with water vapor. The preferred acid gases for the stabilization step are H.sub.2 S, HCl, NO.sub.2 and SO.sub.2 with H.sub.2 S being the most preferred.
The preferred water sensitive polymer for coating the piezoelectric crystals in this invention is sodium polystyrene sulfonate. Of course, if the polymer coating used in the analyzer is a sulfonic acid salt prior to the stabilization step, the polymer can be converted to a sulfonic acid. If the inorganic salt formed as a result of this reaction is not volatile, it will remain on the surface of the polymer.
The loss of sensitivity appears irreversible and must be compensated for in the calibration of the moisture detector. However, the stable sensitivity, even though it is lower, permits accurate measurement of moisture in gas. Such accurate measurements are not possible so long as the sensitivity of the instrument is unstable.
In a preferred method for preconditioning the coated piezoelectric crystal, the coated crystal is exposed to a high concentration of the acid gas or the pure gas containing moisture. In the case of H.sub.2 S, preconditioning a crystal coated with sodium polystyrene sulfonate by contacting it with pure H.sub.2 S saturated with water for about 36 hours at room temperature reduces the sensitivity of the coated crystal to moisture to a stable sensitivity level about 50% less than the original sensitivity. The same procedure can be used with acid gases such as HCl or moist SO.sub.2 or NO.sub.2.
Since the moisture analyzers of this invention are particularly useful in acid environments, all of the components of the analyzer which contact the gas streams must be acid resistant. In the most preferred embodiment, the electrode layer on the crystal is gold and the metal used in the conductive adhesive which binds the crystal to its mounting posts is gold.